Generally, it is known that a large magnetizing inrush current will flow when no-load magnetizing is performed by supplying power while residual magnetic flux is present in a transformer core. The magnitude of the magnetizing inrush current becomes several times the rated load current of the transformer. When such a large magnetizing inrush current flows, system voltage fluctuates. If the voltage fluctuation is large, an influence will be given to customers in some cases.
As a method for suppressing the magnetizing inrush current, a suppressing method using a circuit-breaker with a resistor having a turn-on resistor and contact serially connected is provided. The circuit-breaker with the resistor is connected in parallel with the circuit-breaker main contact. The circuit-breaker with the resistor is connected with a power system prior to the circuit-breaker main contact.
Further, there is provided a method for suppressing a magnetizing inrush current by first closing a given one phase and then closing the remaining two phases when a three-phase transformer of a direct grounding system is connected with a power system by use of three single-phase circuit-breakers. With this suppressing method, the magnitude of the residual magnetic flux when the transformer is interrupted is grasped.
Additionally, there is provided a method for calculating residual magnetic flux when a transformer of an ineffective grounding system is interrupted by measuring phase voltages to suppress a magnetizing inrush current of the transformer.
However, the circuit-breaker having the circuit-breaker with the resistor added thereto will become large. Further, when the circuit-breaker is of a three-phase simultaneous operation type, the magnetizing inrush current cannot be suppressed by closing the circuit-breakers for respective phases. In addition, a zero-phase voltage that occurs after interruption of the first phase remains as DC voltage in the transformer after closing of the circuit-breaker and the residual magnetic flux cannot be precisely derived in some cases.